A large variability in the cooking quality of pulses or dhals has been observed among different pulses and even among different varieties of the same pulse. It has always been a difficult task to precisely assess the cooking quality of dhals. Current methods are very subjective. Generally, cooking quality of dhals is a function of cooking time i.e., time of boiling the dhal in excess water until it attains desired soft texture and is ready for consumption. Cooking quality has been shown as a function of water uptake, increase in volume after cooking, dispersibility of solids into water and textural softening during cooking and some of these could be used as indices of cooking. Textural softening and end point of cooking is judged by pressing the cooked dhal between two glass slides and taken as time at which ‘there are no hard-centres’. Obviously this is quite subjective and varies from analyst to analyst and also cannot be easily quantified. Similarly the end point of cooking of rice is normally judged by pressing the cooked rice between two glass slides and the time at which there is disappearance of white core is taken as its cooking time. However, these are quite subjective and do not provide a quantitative method for direct measurement of texture of cooked grains (dhals/rice). Literature available for this and their limitations are reviewed here.
Reference may be made to the publication by Subba Rao et al (1964) who studied the effect of cooking conditions and chemicals on the cooking quality of several dhals. They have described a method for evaluating the cooking quality in terms of cooking time. A suggestion was also made that quantity of solids dispersed during cooking could be used as a measure of cookability of pulses when cooked as dhals. They showed that different dhals dispersed to different extents during cooking and hence could be used as a method for comparing the cookability of different dhals. However, this does not provide a method for measuring the cooked grain texture.
Narasimha and Desikachar (1978) made a study on the cooking quality of dhals from sixteen varieties of tur pulse (Cajanus cajan) in comparison with that of dhals from green gram and horse gram and showed that percent dispersed solids (% DS) during cooking correlated well with cooking time of the dhals. It was suggested that this could be used as an objective measure of cooking quality of tur when cooked as dhal and showed that a measurement of dispersed solids at a particular time of cooking of the dhals could be used for comparing the cookability of different pulses since % DS correlated very well with the cooking time of dhal being cooked. Though this provided a method for comparing the cookability of different pulses when cooked as dhals, it did not provide a method for a direct measurement of this attribute.
Reference may be made to the publication by Yusuf and Tabey (1981) where different methods for measuring cooking quality of faba beans (Vicia faba) were surveyed; They included subjective methods, use of tenderometer, penetrometer and correlate the values measured to cookability. However all these were applicable only for whole grains, based on compression or penetration of cooked grains and correlating the force required to texture in terms of 50% compression of cooked grains. These are not applicable to dhals because of small thickness and difference in the pattern of cooking between whole grains and their Pals.
Reference may be made to a method suggested by Pratape and Narasimha (1994) where the Chopin—INRA viscoelastograph was used to determine firmness of cooked dhal from soya bean. However, this method is more suitable for dhals, which do not loose shape (non-dispersible type) during cooking and hence can be applied to dhals at a stage earlier to full cooking. Moreover the instrument is rather sophisticated, expensive, and not easily affordable for all laboratories in a country like ours and needs expertise to handle. Hence it may not be very much suitable for routine use. In addition, it is based on resistance to compression and elastic recovery on release of the force applied on cooked grains and hence more suitable for whole grains rather than dhals.
Reference may be made to the publication by Matson (1946) wherein a device, named ‘Bean cooker’, for measuring cookability of yellow cowpeas was described. This device consisted of one hundred plunger-based unit wherein one rod was placed on each of the bean sample being cooked and the entire device was kept in a boiling water bath. Number of beans penetrated with progressive cooking was counted and the time at which 50% of the grains were penetrated was taken as cooking time. Limitations of this unit is that many dhals and whole pulses become soft even due to hydration alone and hence even though they had not attained cooked texture, they were easily penetrated. This resulted in wrong judgment of cooking time and hence does not very well meet the requirements of measuring the cooked cotyledon/dhal texture. Chinnan (1985) modified this unit for evaluating the degree of hardness in cooked beans, by introducing an electronic measuring device. However the drawbacks mentioned earlier are still associated with this modified unit also.
Rice is another cereal grain that is extremely used in the grain form after cooking to soft texture. A few attempts have been made for judging the textural softness of cooked rice and these are reviewed here. Reference may be made to the publication by Ranghino, (1966) who has suggested the pressing of cooked rice between two glass slides and observing the time at which there is disappearance of the white core and this is taken as its cooking time. However, this is very subjective (varies from person to person) and does not give a quantitative measure of the texture of cooked rice.
Reference may be made to the publication by Pillaiyar and Mohandoss (1981), who have described an impact-pressing device. Here a mild steel shaft is momentarily dropped from a pre-determined height (10 or 15 cm) on to a glass plate where the cooked rice is kept and the resulting pressed area is measured using a planimeter. Pressed area was determined for differently prepared parboiled rice sample and correlated to their sensory characteristics. They showed a relationship between the pressed areas of cooked rice to L/B ratio of the corresponding raw grains and suggested that this method could be used for differentiating differently processed parboiled rice samples. These authors, however did not suggest a method for an objective measurement of cooking time of rice being cooked. Moreover the impact system used here caused extensive mechanical damage to the cooked grains and could not be applied for judging the end point of cooking.
Sowbhagya and Ali (1990) have suggested that cooked rice attains final moisture of around 72% (wb) and could be well correlated with firmness (% F) and expansion ratio measured in a viscoelastogram. It was suggested that the grain could be considered “cooked” when it attains an F value of around 70%. This method was shown to be applicable to measure the firmness of cooked soya bean by Pratape and Narasimha, (1994). However the tests are sophisticated and may not be suitable for routine use due to non-availability of the instrument.
Need for the invention of a simple device and an objective method for judging the end point of cooking of grains like rice and dhals that could be used for routine evaluation therefore exists. Most of the methods described earlier do not give an objective and direct method for measuring the textural softness (doneness or end point of cooking) of cooked grains (dhals and rice) and current invention fulfills this gap.